The present invention relates to the digitalization of a subscriber's telephone and more particularly to the supply of such a telephone having regard to the various basic and specific functions attributed thereto.
The basic functions essentially intended to assure the telephone service comprise in particular the supply, call, conversation, reception of call. The specific functions include functions for the telephone service: for example, loudspeaker reception, dialling without lifting the receiver, reply service, dialler of numbers and functions for the new services such as the data transmission, telemetry, remote-alarm services.
To perform these different functions, a terminal, hereinafter called a subscriber control system, allows the telephones and telematic terminals capable of functioning simultaneously, to be connected. The subscriber control system may, for example, be connected to the exchange by means of a two-wire circuit. Such a two-wire circuit makes it possible to arrange high speed data channels in the two directions of transmission.
For known subscriber's telephones of analog type, a central battery supply enables the telephone to operate. Monitoring of the supply current of the telephone allows supervision of the loop state of the subscriber in order to detect, at exchange level, that the subscriber who wishes to make a communication has lifted his receiver (outgoing call). In the case of an incoming call, the subscriber is alerted by the emission of a call current at 50 Hz or 25 Hz which actuates a bell located in the subscriber's premises.
A supply system of central battery type may also be envisaged for the subscriber control system. In the event of the receiver being lifted (outgoing call), it should then allow loop closure. In the event of an incoming call, the conventional method of the signal at 75 V eff (25 or 50 Hz) capable of activating a local bell does not, however, seem to be very compatible with the digital signals transmitted on-line. In fact, the digital signals are of lower amplitude than the analog signals and require considerable protection. The high value of the voltage of the call signal may consequently raise serious problems of protection and interface.
It is an object of the present invention to propose a central supply of the subscriber control system at least as far as the basic functions (telephone) are concerned, and to allow the outgoing or incoming communications to be initialized.
The system being supplied, a simple solution enabling the outgoing and incoming communications to be initialized would consist in transmitting messages between control system and centre of connection of the exchange on the digital information channel.
Such a solution is extremely interesting as it enables the exchange to supervise the state of the subscriber's terminals and in particular to detect at exchange level that the subscriber who wishes to make a communication has lifted his receiver. Thus, the state of the subscriber's telephone may easily be transmitted with the aid of the information output intended for signalization.
However, such a solution would be very expensive energy-wise as it would require a permanent supply of all the subscribers, in order to guarantee the possibility at any moment of signalization information transmission between control system and exchange. Moreover, the risk of disturbance between systems would increase as all the systems would be active simultaneously.
The present invention aims on the one hand at ensuring a supply of the subscriber's telephone from the exchange, and on the other hand at providing this supply solely in response to calls emitted either by the exchange or by the subscriber, to initialize a communication. In the case of an incoming call, in order to initialize the communication, the exchange supplies the control system. When a so-called outgoing call is emitted by a subscriber's telephone towards the exchange, it is necessary to warn the exchange to supply the control system. It is an object of the present invention to generate a so-called preamble message from the control system to warn the exchange to ensure normal supply of the control system.